1. Field of the Invention
The present invention relates to a photovoltaic cell which has a transparent coating film on a light receiving face of a photovoltaic element for preventing photovoltaic element properties from being degraded by scratches, moisture, and the like.
2. Description of the Related Art
In recent years, a greenhouse effect, that is, global warming caused by increase in amount of CO2, has became a serious problem, and hence development of clean energy sources which will not discharge CO2 at all has been increasingly required. As one of the energy sources described above, for example, nuclear power generation may be mentioned; however, since various problems thereof, such as radioactive wastes, must be solved, development of safer clean energy sources has been further required.
Accordingly, among various clean energy sources which are expected in the future, solar cells have drawn significant attention because of their cleanness, highly reliable safety properties, and easy handling.
At present, various solar cells have been proposed, and some of them have actually been used as an electrical source. The solar cells mentioned above are roughly categorized into a crystal silicon-based solar cell using single crystal or polycrystal silicon, an amorphous silicon-based solar cell using amorphous silicon, and a compound semiconductor solar cell.
In general, a solar cell is used as a solar cell module in which a photovoltaic element is covered with a transparent coating material for protection. This surface coating material provided at a topmost surface is composed of a glass or a transparent fluorinated resin formed of a fluorinated resin film, a fluorinated resin paint, or the like, and a seal composition material is provided thereunder. In general, as the seal composition material, various transparent thermoplastic resin compositions are used.
The reasons a glass substrate is used at the topmost surface are that a glass substrate has superior moisture resistance, scratch resistance, and weather resistance and that a decrease in conversion efficiency of a solar cell module which is caused by decrease in transmittance due to degradation can be suppressed. In particular, as for scratch resistance, a glass substrate may be regarded as one of the most superior materials since it mechanically protects a photovoltaic element without causing any damage thereto. However, a glass substrate has several problems, such as heavy weight, inflexibility, poor impact strength, and high cost, and in particular, the heavy weight has serious influence on an installation structure of the solar cell module.
In addition, due to various installation structures of solar cell modules and output requirements of current, voltage, and the like, the external form of a solar cell module is considerably changed. On the other hand, in general, several types of external forms of photovoltaic elements have been standardized, and in accordance with the structure of a solar cell module, a plurality of photovoltaic elements are connected to each other to form a desired structure. That is, depending on the production schedule or the like for a solar cell module, photovoltaic elements may be temporarily stored or transported in some cases after being formed.
When being stored or transported as described above, the photovoltaic element is not covered with a coating material, and hence properties of the photovoltaic element are degraded by moisture and/or scratches, thereby causing decrease in production yield. For protection of photovoltaic elements in a production process, as disclosed, for example, in Japanese Patent Laid-Open Nos. 9-36396, 9-92759, and 10-233519, a thin film resin layer used as a protection coating may be provided for the photovoltaic element immediately after the production thereof to form a photovoltaic cell.
A typical photovoltaic element structure has a semiconductor layer having a pn junction; a light receiving electrode made of a transparent conductive oxide provided on a light receiving face of the semiconductor layer; a collector electrode which is formed of a relatively thin metal for collecting current and which is provided on the light receiving electrode; and an electrode formed of a relatively thick metal called a bus bar for collecting current collected by the collector electrode.
As an electrode structure of a photovoltaic element, for example, according to U.S. Pat. No. 4,260,429, an electrode has been proposed which is composed of a polymer containing conductive particles and a metal wire coated therewith. According to this patent, since a metal wire made of copper or the like having superior conductivity is used, electrical resistance loss can be reduced even when a long collector electrode is formed, and in addition, since an aspect ratio of 1 to 1 can be realized, the shadow loss can also be reduced. In addition, according to this patent, by using a conductive adhesive for fixing the wire, bonding can be advantageously performed by simple thermo-compression bonding. The inventors of the present invention improved the collector electrode made of the metal wire described above and proposed an electrode structure of a photovoltaic element as disclosed in Japanese Patent Laid-Open Nos. 7-321353, 9-18034, and 10-65192.
One example of a method for forming the electrodes of a photovoltaic element will be described with reference to FIGS. 5A and 5B. FIGS. 5A and 5B are schematic views each showing the structure of a photovoltaic element having collector electrodes using metal wires; FIG. 5A is a schematic view of the photovoltaic element when viewed from a light receiving face side; and FIG. 5B is a schematic view of the photovoltaic element when viewed from a non-light receiving face side.
In FIGS. 5A and 5B, reference numeral 501 indicates a photovoltaic element plate composed of a substrate and three layers provided thereon, the three layers being a lower electrode layer, an amorphous silicon layer responsible for a photovoltaic function, and an upper electrode layer. The collector electrodes are to be formed on this photovoltaic element plate 501.
In this photovoltaic element plate 501, aluminum (Al) and zinc oxide (ZnO) are formed in that order on a stainless steel substrate supporting the entire photovoltaic element plate 501 by sputtering so that each has a thickness of several hundreds nanometers, thereby forming the lower electrode layer. In addition, an n-type, an i-type, a p-type, an n-type, an i-type, and a p-type layer are deposited in that order from the substrate side by plasma CVD to form the amorphous silicon layer. In addition, the upper electrode layer is formed of a transparent electrode film, and an indium oxide thin film is formed by depositing indium (In) in an oxygen atmosphere by resistance heating.
Furthermore, in order to avoid the adverse influence of short circuiting between the substrate and the transparent electrode film on an effective light receiving area, which short circuiting occurs when the outer periphery of the photovoltaic element plate 501 is cut, after an etching paste containing FeCl3, AlCl3, or the like is applied onto the transparent electrode film by screen printing, followed by heating, washing is performed so that parts of the transparent electrode film of the photovoltaic element plate 501 are linearly removed, thereby forming etching lines 502.
Subsequently, copper foil strips used as rear-side conductive foil bodies 503 are formed along two sides of the non-light receiving face side of the photovoltaic element plate 501 by a method disclosed in Japanese Patent Laid-Open No. 8-139349.
Next, onto two sides facing the rear-side conductive foil bodies 503 provided on the non-light receiving face of the photovoltaic element plate 501, insulating member 504 composed of a base plate made of polyimide and acrylate adhesives provided on both sides thereof are adhered.
Next, metal wires 505 each formed of a copper wire coated with a conductive adhesive composed of a carbon paste beforehand are continuously formed on the photovoltaic element plate 501 and the insulating members 504 at predetermined intervals, thereby forming the collector electrodes.
Furthermore, on the insulating members 504, conductive foil bodies 506 are formed which are used as additional collector electrodes for the collector electrodes described above. In particular, after copper foil strips are placed, the entirety is then fixed by applying heat and pressure thereto under predetermined conditions.
On the light receiving face of a photovoltaic element 500 formed in accordance with the steps described above, surface protection coating is performed. One example in which a surface protection coating film is provided for the photovoltaic element 500 which has the collector electrodes made of the metal wires will be described with reference to FIG. 6. FIG. 6 is a schematic view showing a photovoltaic cell composed of the photovoltaic element and a coating film provided thereon when it is viewed from the light receiving face side. In FIG. 6, reference numeral 601 indicates a surface protection coating film which is formed by applying a predetermined coating solution by spraying coating to a power generation region of the photovoltaic element, followed by heating for curing.
By the method described above, a photovoltaic cell 600 can be formed. In the photovoltaic cell 600, even on the light receiving face of the photovoltaic element, for example, the coating film 601 is not applied to the conductive foil bodies 506 in order to mount backflow prevention diodes or the like thereon in a subsequent step. That is, the surface protection coating film is provided only in the power generation region of the photovoltaic element.
Objects of the present invention will be described with reference to FIG. 7. FIG. 7 is a schematic cross-sectional view of the photovoltaic cell taken along the line 7-7′ in FIG. 6.
On the light receiving face of the photovoltaic element described above, the insulating members 504, the conductive foil bodies 506, and the like are placed, and in the power generation region of the photovoltaic element, the surface protection coating film 601 is provided. In the step described above, since the heights of the insulating member 504, the conductive foil body 506, and the like are larger than the average height of the surface protection coating film 601, due to the surface tension or the like, the thickness of the surface protection coating film (indicated by the arrow in FIG. 7) in the vicinity of the insulating member 504 becomes considerably larger than the average thickness described above.
In the state described above, when the surface protection coating film 601 is dried for curing, a problem occurs in that internal bubbles are generated in the surface protection coating film 601 in the vicinity of the insulating member 504. That is, since drying conditions for curing the surface protection coating film 601 are set in consideration of the average film thickness thereof, at a part of the surface protection coating film 601 having a larger thickness, a solvent contained therein cannot be removed by evaporation within a predetermined drying time. Hence, it is believed that even after the curing of the coating film has started, the solvent is still being evaporated, and as a result, internal bubbles are generated in the surface protection coating film 601.
Since the internal bubbles described above are generated in the power generation region of the photovoltaic cell, optical properties are degraded, the properties of the photovoltaic cell are degraded, and in addition, due to the cosmetic defects, the yield is also reduced.
As a method for solving the problems described above, it may be considered that the drying time is increased; however, in order to prevent the generation of internal bubbles at a place at which the thickness of the coating film is large, the drying time must be several times that performed in the past, and in addition, the drying temperature must be precisely controlled. Accordingly, in the case of a photovoltaic cell having a large area such as 200 mm by 250 mm, the method described above cannot actually solve the above problem. In addition, when the drying time is greatly increased, yellow discoloration of the coating film may occur, thereby causing degradation of optical properties such as transmittance.